Many membrane processes are temperature-sensitive. For example, the selectivity of a gas-separation membrane for one gas over another often increases as the temperature of the gas decreases. On the other hand, the feed gas may need to be warmed to keep water vapor or other condensable components from liquefying within the membrane module. The performance of a pervaporation system is usually improved by heating the feed liquid, because this raises the vapor pressure on the feed side, increasing the transmembrane driving force and hence the transmembrane flux. In ultrafiltration, it may be necessary to keep the liquids under treatment within a specific temperature range, to prevent spoilage of food products, for example. In hemodialysis, careful temperature control is clearly extremely important. In any membrane process, the feed stream may contain labile compounds that must be kept cool. Conversely, solutions may contain solutes that will crystallize out unless the solution is kept above certain temperature.
It is apparent that there are numerous situations where an ability to warm or cool the feed stream is useful. This can, of course, be achieved by direct or indirect heating or chilling at some point upstream from the membrane module, but this may necessitate insulation of feed lines and membrane system to maintain the desired temperature during the membrane separation operation.